1. Field of the Invention
The present invention relates generally to current-interrupting devices for electrical power distribution systems and more particularly to a low-exhaust composite drop-out fuse assembly that is usable with a standard cutout mounting, provides current-limiting characteristics, and includes a low-current clearing section that is simply and economically renewable for reuse.
2. Description of the Related Art
Various assemblies and devices have been proposed to provide current interrupters in electrical power distribution circuits that have reduced exhaust characteristics as compared to distribution cutouts with replaceable fuse links. The class of expulsion fuses known as cutouts have undesirable exhaust characteristics that are well known in the industry and discussed in the literature, e.g. see ANSI/IEEE standard C37.48 and the U.S. Patents discussed herein. One approach to reduce the exhaust of expulsion devices as set forth in U.S. Pat. No. 3,863,187 provides the series connection of a current limiting fuse with a conventional cutout. The current limiting fuse is connected to one of the terminals of the cutout mounting and is not part of the drop-out fuse tube assembly. While this approach does reduce the exhaust characteristics of the cutout and also provides current limiting of the faults in the circuit, it also has undesirable characteristics regarding installation and maintenance. Specifically, the current limiting fuse is difficult to replace from a distance which requires the lineman to either deenergize the lines or come in close proximity to energized lines. Obviously, since safety of operating personnel is an important essential of any product use, neither alternative is readily acceptable. Since there is no provision to easily determine whether or not the current limiting fuse has operated, this requires the difficult servicing procedure every time that the cutout operates and drops out.
Other approaches are directed to the provision of a composite or combined assembly some of which are of the drop-out type. These composite assemblies provide a single assembly or device that is the combination of a current limiting fuse with another lower-current fuse or cutout. For example, approaches of this type are shown in U.S. Pat. Nos. 3,893,056, 5,274,349, 4,011,537, 3,827,010, 4,184,138, 4,114,128, 4,121,186, 4,161,712 and 2,917,605.
The arrangement of U.S. Pat. No. 3,893,056, which is not directed to a drop-out assembly or a disconnect mounting, utilizes the interior of the current limiting section to accommodate operational portions of the expulsion fuse section such as the arcing rod or a muffler portion. However, the fuse sections are not separable and the expulsion fuse is a one-shot device that does not have provisions for refusing.
A current-limiting drop-out fuse is shown in U.S. Pat. No. 5,274,349. This arrangement is not vented (no exhaust provisions) and also includes a low-current clearing section. Unfortunately, the entire one-piece fuse body must be replaced after all types of operation, i.e. even after clearing low-range overcurrents.
The other aforementioned composite or combined assemblies (e.g. as shown in the aforementioned U.S. Pat. Nos. 4,011,537, 3,827,010, 4,184,138, 4,114,128, 4,121,186, 4,161,712 and 2,917,605) also provide some desirable features but suffer from one or more drawbacks. For example, they all utilize expulsion fuse sections of the cutout type that have the undesirable exhaust characteristics as a result of open-ended fuse tubes through which a fuse-links cable exits. Further, regarding mechanical configuration, these arrangements all have the problem of fitting the cutout fuse tube and the current-limiting fuse within the straight line distance between the mounting terminals. While the composite fuse assembly of the aforementioned U.S. Pat. No. 4,184,138 provides a cutout fuse tube at an angle to the current-limiting fuse such that the combined lengths of the two sections may be somewhat greater than the straight line distance, this configuration still results in a very short length that is available for the cutout fuse tube. The remainder of the aforementioned arrangements utilize in-line configurations of the cutout fuse tube and the current-limiting fuse such that the available length must be divided between the two devices. Additionally, some of the aforementioned arrangements are not usable with existing standard cutout mountings which would require the purchase and installation of new mountings throughout a distribution system.
While the prior art arrangements may be useful to provide combinations of current interrupting devices with other devices, none of these previous approaches provides a desirable commercial replacement for a distribution cutout, namely a drop-out assembly with low exhaust characteristics that is usable in a standard cutout mounting and that allows the current-limiting section to be reused when only the low-current section has operated.